PROJECT SUMMARY/ ABSTRACT This multidisciplinary, 5-year K08 application is designed to facilitate the career development of Dr. Judith F. Ashouri, MD, an Assistant Professor in Rheumatology at UCSF, as an independent physician-scientist. Dr. Ashouri specializes in rheumatoid arthritis (RA). After receiving training in Rheumatology and Immunology at UCSF, she now seeks to further develop her research experience to attain her long-term goal of becoming an independent investigator, dissecting the contribution of antigen (Ag)-specific, pathogenic CD4 T cells to arthritis development in RA. This K08 award will provide Dr. Ashouri with the support and protected time necessary to accomplish the following objectives: (1) to improve advanced immunology skills including immunophenotyping using mass cytometry and T cell receptor (TCR) sequencing,(2) to become proficient in advanced statistical analyses using RNAseq data sets, (3) to acquire skills critical for an academic career including scientific writing and oral presentation, and (4) to submit an R01 application during year 5 of the proposed grant. The candidate has assembled an exceptionally strong advisory team of experts in Basic/Translational Immunology and Autoimmunity?all of whom have successfully mentored many young scientists. Her career development plan includes regular meetings with her mentors, didactics, and attendance at local and national meetings. UCSF, a world-class institution with particular strengths in immunology, translational research, and autoimmunity, is the ideal environment for developing her career. RA afflicts millions globally and is without cure. CD4 T cells are known to play a key role in RA pathogenesis. Yet, it remains unknown how arthritis-causing T cells initiate disease and to what Ag these T cells respond. T cells can either be activated through their TCR in an Ag-specific manner, or in response to cytokines. Identification of Ag-activated T cells in RA would allow us to investigate arthritogenic clones and elucidate early events in disease pathogenesis. However, isolating relevant arthritogenic TCR-activated T cells (as opposed to `bystander' T cells activated by the inflammatory milieu) has limited the field's understanding of disease-initiating events in RA. Dr. Ashouri and colleagues have pioneered a new strategy to overcome this limitation. In this proposal, she builds on preliminary data that demonstrate the ability to track Ag-specific, arthritogenic T cell responses in RA in both a mouse model of RA (SKG mice) and human RA joint tissue. Tracking these arthritis causing T cells will facilitate Dr. Ashouri's (a) short term goal to identify and study arthritis-causing T cells before and during RA disease development and (b) long term goal to identify the inciting Ag(s). Successful completion of the proposed studies will provide new mechanistic insights of how arthritis-causing CD4 T cells contribute to destructive RA and holds promise for improved therapeutic targets.